[ADT] Replace APInt::WORD_MAX with APInt::WORDTYPE_MAX
The windows SDK defines WORD_MAX, so any poor soul that wants to use LLVM in a project that depends on the windows SDK gets a build error.
Given that it actually describes the maximal value of WordType, it actually fits even better than WORD_MAX
Patch by: @miscco
Differential Revision: https://reviews.llvm.org/D50777